The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the production of MRI images based on the measurement of blood volume.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The NMR signal which is emitted by tissue after excitation decays in amplitude at a characteristic rate referred to as the T.sub.2 relaxation rate. The T.sub.2 relaxation rate of a tissue is determined by a number of factors, including its magnetic susceptibility. There are many NMR pulse sequences which are particularly sensitive to differences in the T.sub.2 relaxation rate of tissues, and these are employed to produce images in which the contrast between these tissues is enhanced. For example, the T.sub.2 time of cerebral fluid is significantly different than that of gray matter and excellent MRI images of the brain can be obtained with T.sub.2 weighted MRI scans.
While conventional T.sub.2 weighted images of the brain will often differentiate tumors from normal brain tissues, they do not provide sufficient clinical information. For example, such images do not differentiate between benign and malignant tissue, or between radiation necrosis and tumor recurrence, or (in a malignant tumor containing heterogeneous tissue types) between low grade and high grade malignant tissues. As a result, other imaging modalities such as x-ray angiography, positron emission tomography (PET) or single-photon emission computed tomography (SPECT) have been used to obtain the additional clinical information needed to diagnose and treat brain tumors.
Recently, dynamic susceptibility contrast MRI methods have been developed to study brain functions and to assess tumor vascularity. These methods employ a magnetic susceptibility contrast agent which is injected into the patient's vasculature during the acquisition of a series of MRI images of the brain. A reduction in NMR signal intensity occurs when the susceptibility contrast agent flows into image voxels. The overall decrease in signal intensity within each voxel as the "bolus" of susceptibility contrast agent flows through the brain is a measure of the blood volume within each voxel. An image based on these blood volume measurements thus contrasts those regions of the brain which contain different proportions of blood per unit volume and provides valuable clinical information regarding brain tissue health.
The quality of the image produced using dynamic susceptibility contrast methods is determined largely by the accuracy with which blood volume in each voxel can be estimated. Since the NMR signals themselves are very small relative to system noise, and the "dip" in amplitude due to the susceptibility contrast agent is even smaller, the results achieved by simply integrating the measured dip over the time interval in which the bolus passes does not produce an accurate result. Also, although the values that would be produced for each voxel by integrating the image intensities over time may be in some sense qualitatively similar to the cerebral blood volume, they would not be quantitatively proportional to the cerebral blood volume in each voxel. In addition, the results can be adversely affected by recirculation of the susceptibility contrast agent through the voxels while contrast from the initial bolus is still present.